1. Field of the Invention
The invention generally relates to glass wafers. More particularly, the invention relates to glass wafers made of infrared absorbing glasses.
2. Description of Related Art
As is known, camera chips typically have the property that the pixels of the chip are sensitive also in the infrared spectral range. Moreover, the optical system of camera modules whose optical components are made from standard glasses or plastic materials generally exhibit a certain amount of infrared transmission. However, infrared light that reaches the chip results in undesirable color and brightness distortions.
For this reason, camera modules are typically equipped with infrared filters. The most common infrared filters are interference filters. For such filters, a multi-layered dielectric layer system is deposited on a substrate, typically a glass substrate. The multi-layered dielectric layer system, based on physical reasons, is designed to reflect infrared radiation, but to transmit visible light. Such filters are relatively inexpensive to produce, but have several drawbacks. Interference filters often impart a certain modulation to the transmission curve. This modulation has an effect similar to that of a comb filter and may affect individual colors.
Moreover, interference filters exhibit a much stronger dependency of the filter curve (transmission curve) from the light incident angle than optical filter glass which is also referred to as “colored glass” or as absorption filter. Compact cameras typically have a full opening angle of up to 30° and often are not telecentrically aligned, i.e. the light rays impinge to the image sensor at a certain angle (with the full opening angle).
Additionally, the infrared light is reflected back by the interference layer into the optical system. Since the interference filter generally still exhibits a residual transmission at least in the near infrared range, very annoying ghost images may occur in the optical system due to multiple reflections.
An alternative thereto is provided by infrared filters in form of filter glasses. A filter glass, by virtue of its character neither exhibits the aforementioned comb filter effect nor ghost images due to multiple reflected infrared light, since the infrared light is absorbed when passing through the glass.
However, heretofore, their cost-efficient manufacturing is not the only advantage of interference filters when compared to filter glasses. The interference layers are very thin and can be deposited on very thin substrates. This has so far allowed to produce more compact optical systems using interference filters.